This invention relates in general to thin-film, infrared, photoconductive detectors and, more particularly, to the hardening of such detectors against damage resulting from laser irradiation.
The threat of laser countermeasures against military electrooptical systems has created a need for infrared detectors hardened against damage from intentional laser irradiation. Unintentional irradiation may also damage the detectors. Infrared detectors of the type that use a thin film of photoconductive material as the active detector medium are widely used for various heat and object-sensing applications. In particular, room-temperature, lead sulfide (PbS), thin-film, photoconductive detectors are a popular choice for sensor systems operating in the 1-3 .mu.m atmospheric spectral window.
Although much effort has been given to sensitizing thin-film detectors, they have not been designed for optimum heat transfer. Consequently, their ability to dissipate absorbed power is usually quite poor and they are susceptible to laser damage.
The thin film of photoconductive material (such as a thin film of PbS, typically 1 .mu.m in thickness) must of necessity be deposited on a substrate to provide mechanical support and stability. (It must also be non-conducting to provide electrical isolation for the film.) This substrate, of course, normally provides some heat-dissipation capability for the detector. However, in the area of temperature control, the main concern in prior-art thin-film detectors has been the maintenance of the photoconductive film at a temperature at which the detector will function most efficiently, that is, maintaining a temperature at which there is a high photoconductive gain. Thus, the prior-art detectors are designed to remove heat generated by biasing currents, and normal non-intensive radiation and thus maintain normal operating temperatures.
However, intensive laser irradiation causes much higher thermal loading which may raise the temperature to the point where permanent degradation or complete loss of detector responsivity results if the detector cannot dissipate the incident energy. For example, it has been found that the onset of damage may occur in thin-film, PbS, photoconductive detectors when the detector is heated to approximately 600.degree. K. by laser irradiation.